1. Field of the Invention
The present invention relates to a secondary side synchronous rectification control circuit and a switching converter; in particular, to a secondary side synchronous rectification control circuit and a switching converter having an inverted amplifier.
2. Description of Related Art
FIG. 1 shows a circuit diagram of the conventional flyback converter circuit. As shown in FIG. 1, the flyback converter circuit has a transformer T, a primary side switch Q1, a pulse width modulation controller 10, a secondary side synchronous rectification switch Q2, a secondary side synchronous rectification control circuit 20. The pulse width modulation controller 10 generates a driving signal Vgs1 according to a feedback signal from the secondary side of the transformer T, for controlling the conduction cycle of the primary side switch Q1. The secondary side synchronous rectification control circuit 20 detects the drain source voltage Vds2 of the secondary side synchronous rectification switch Q2, for controlling the conduction cycle of the secondary side synchronous rectification switch Q2.
FIG. 1A shows a circuit diagram of the secondary side synchronous rectification control circuit 20 in FIG. 1. FIG. 2 shows a waveform diagram corresponding to the working signals in FIGS. 1 and 1A. As shown in FIG. 1A, the secondary side synchronous rectification control circuit 20 has a comparator 22 and a driving unit 24. An input end of the comparator 22 is for detecting the drain source voltage of the synchronous rectification switch, and the other input end is for receiving a reference voltage Vr, in order to generate a control signal to control the conduction cycle of the secondary side synchronous rectification switch Q2.
Generally, the conduction resistance RDS(on) of the power transistor applied to the synchronous rectification operation is very low, which only has the value between several mΩ to tens of mΩ. Thus, during the conducting period of the secondary side, the drain source voltage Vds2 cause by currents flowing through the secondary side synchronous rectification switch Q2 is very low. In addition, because of the influences of the input offset voltage of the comparator, the turn-off timing of the driving signal Vgs2 may be varied. If the turn-off timing of the driving signal Vgs2 is too early, the secondary side synchronous rectification switch Q2 is turned off early, which influences the efficiency. If the turn-off timing of the driving signal Vgs2 is too late, the secondary side synchronous rectification switch Q2 may generate current flowing backward, which generates abnormal spike voltage and extra power loss. Moreover, if the spike voltage exceeds the designed specification of the secondary side synchronous rectification switch Q2, it may also damage the secondary side synchronous rectification switch Q2.